The present invention relates to a process for the fabrication, on a plasma display panel sheet, of a dielectric layer with raised designs. The application of the invention is particularly advantageous when used with plasma panels of the alternative type.
Plasma panels (abbreviated to xe2x80x9cPPxe2x80x9d in the rest of this description) are image display screens of the xe2x80x9cflat screenxe2x80x9d type that operate using the principle of a discharge in gases.
PPs generally include two insulating sheets, each one carrying one or more networks of electrodes, the space between the sheets being gas-filled. The sheets are assembled together such that their networks of electrodes are mutually orthogonal. Each intersection of electrodes defines a cell corresponding to a gas space in which an electric discharge is produced when the cell is activated.
FIG. 1 represents by way of example, in a partial and simplified manner, a classic structure of a color alternative PP. Various types of alternative PPs are found, among which we can mention for example: those of the type using only two crossed electrodes to define and control a cell, as described notably in a French patent no. 2 417 848, and those of the so-called xe2x80x9ccoplanar structurexe2x80x9d type whose structure and operation are described for example in the European patent EP-A- 0.135.382. Alternative PPs have a common characteristic, in that they have an internal memory effect in operation, owing to the fact that their electrodes are separated from the gas and the discharge by a layer of dielectric material.
In the example of FIG. 1, the PP is of the type having two crossed electrodes defining a cell. It is composed of two substrates or sheets 2, 3, of which one is a front sheet 2, i.e. the sheet that is on the same side as an observer (not shown); this sheet carries a first network of electrodes called xe2x80x9cline electrodesxe2x80x9d, of which only 3 electrodes Y1, Y2, Y3 are shown. The line electrodes Y1 to Y3 are covered with a layer 5 of a dielectric material. The second sheet 3 forms the rear sheet. It is on the opposite side from the observer and carries a second network of electrodes called xe2x80x9ccolumn electrodesxe2x80x9d, of which only 5 electrodes X1 to X5 are shown. The two sheets 2, 3, are of a same material, generally glass; they are destined to be assembled together such that the networks of line and column electrodes are mutually orthogonal.
On the rear sheet 3, the column electrodes X1 to X5 are positioned at intervals of P, whose value (for example from 100 xcexcm to 500 xcexcm) depends on the definition of the image. They are they also covered with a layer 6 of dielectric material, whose thickness e1 is generally about 20 xcexcm to 30 xcexcm. In the example shown, the dielectric layer 6 is itself covered with layers of luminiferous materials forming bands 7, 8, 9 that correspond for example to the colors green, red and blue respectively. The rear sheet 3 also includes a network of barriers 11, parallel to the column electrodes X1 to X5 and to the luminiferous bands 7 to 9. These barriers 11 are placed between adjacent luminiferous bands so as to separate them.
The PP is formed by the assembly of the front and rear sheets 2, 3, this operation forming a matrix of cells C1 to Cn. The cells are defined at each intersection between a line electrode Y1 to Y3 and a column electrode X1 to X5, and each cell has a discharge zone whose section corresponds substantially to so-called xe2x80x9cusefulxe2x80x9d areas formed by the surfaces facing the two crossed electrodes. The cells C1 to Cn are illustrated in the figure by cavities Ep1 to Epn made in the luminiferous bands 7 to 9. In the example shown, intersections made by the first line electrode Y1 with the column electrodes X1 to X5 define a line of cells C1 to C5, illustrated respectively by cavities Ep1 to Ep5. For each cell, the discharge in the gas causes electric charges to cumulate on the dielectrics 5, 6 facing the line and column electrodes, in other words at the positions of the cavities Ep1 to Epn.
The dielectric layers 5, 6 therefore have a particularly important function. They are generally made by stoving a glass frit: the stoving increases the density of the material until a glass is formed. Unfortunately, this method frequently leaves defects in the glass, such as bubbles and depressions (resulting in insufficient thickness), or even holes. The ability of the dielectric to hold electrical tension is weakened at these defects. In the case for example of the dielectric layer 6, the dielectric must have breakdown voltages of a few hundred volts.
Another difficulty in the fabrication of PPs is the making of the barriers 11. These barriers commonly act as spacers: they determine the separation distance between the front sheet 2 and the rear sheet 3. This spacing distance is then defined by the height H1 of the barriers 11, commonly from 50 xcexcm to 150 xcexcm depending on the applications. This requires a high level of precision in the height H1 in order to assure optimal discharge characteristics, and very small dispersion in the value of the heights H1 of the different barriers. The barriers must also have a suitable geometry in order to enhance the luminous efficiency of the structure. We note that the barriers 11 may also have another function known as xe2x80x9cconfinementxe2x80x9d, assuring that the cells are xe2x80x9cisolatedxe2x80x9d from each other.
These various criteria are difficult to respect using classical methods of fabrication.
FIG. 2 represents a barrier made in the classic manner by superposed layers: a sheet 20 carries electrodes 21 which are themselves covered with a dielectric layer 22; a barrier 23 is formed on the layer 22 by means of a number N of successive serigraphy operations, each one producing a layer Cs1, . . . , CsN; the number N may be for example between 10 and 20, depending on the height H1 required.
One disadvantage of this method is the large number of serigraphy operations necessary to obtain the height H1. Another disadvantage is the irregular profile of the sides of the barrier 23 which is due to the impossibility of perfectly superimposing the successive layers Cs1 to CsN. Finally, another disadvantage is that it is difficult to achieve the required precision in the height of the barrier, since the layers Cs1 to CsN are not of uniform thickness.
Another classic method of fabrication of the barriers makes use of blasting operations (not shown). This consists in protecting with a mask the zones that are to form the barriers, then blasting the surface to erode the material in the unprotected zones. One of the problems of this method is that the geometry of the barriers is limited, notably the sides are necessarily entirely vertical, which does not favor high luminous efficiency. Another drawback is the risk of damage to the underlying dielectric layer during the blasting operation, which imposes many particularly onerous precautions. Finally, a serious disadvantage of this method is that it makes use of large quantities of grit contaminated by heavy metals contained in the layers subject to blasting, and which must therefore be reprocessed.
The present invention proposes a process that enables a dielectric layer with raised designs, such as for example the network of barriers described previously, to be made on a PP sheet, in a simple manner and avoiding the defects and disadvantages mentioned previously.
The invention is therefore a process for fabrication of a dielectric layer having raised designs on a plasma display panel sheet, consisting in depositing on the sheet a layer containing a glass frit, then vitrifying this layer which becomes a xe2x80x9cvitreous layerxe2x80x9d, characterized in that a mold carrying raised designs is then brought to bear on the vitreous layer, this mold and the sheet bearing the vitreous layer being heated until a creep effect occurs in the vitreous layer enabling it to adopt the shape of the mold.
The term xe2x80x9craised designxe2x80x9d with respect to the surface of the dielectric layer is understood to refer both to projecting elements forming protuberances and projections, like the barriers 11, and to hollow elements like the cavities Ep1 to Epn, for example.